1. Field of the Invention
The present invention generally relates to a touch display panel, and more particularly, to a built-in touch display panel.
2. Description of Related Art
In recent years, along with the rapid developments of the various applications of information technology, wireless mobile phones and information household appliances, to achieve the goals of more convenient usage, more compact design and more humanized features, many information products have changed their input devices from traditional keyboard or mouse to touch display panel.
FIG. 1 is a sectional view diagram of a conventional touch display panel. Referring to FIG. 1, a conventional touch display panel 100 includes a first substrate 110, a second substrate 120 and a liquid crystal layer 130. The second substrate 120 has a plurality of main spacers 128a, a plurality of sensing protrusions 128b, a plurality of sub-spacers 128c and a plurality of second electrodes 126, all of which are disposed thereon. The first substrate 110 has a plurality of padding structures for sensing 114b thereon. Each of the padding structures for sensing 114b has a first electrode 116 thereon. The main spacer 128a is disposed mainly for keeping a certain cell gap between the second substrate 120 and the first substrate 110. The sub-spacer 128c is for assisting support and avoiding the damages of the main spacer 128a and the sensing protrusion 128b caused by excessive deformations when an external force is applied onto the touch display panel and the external force is far greater than the endured load of the main spacer 128a and the sensing protrusion 128b. 
Normally, the first electrode 116 on the padding structure for sensing 114b and the second electrode 126 on the sensing protrusion 128b do not directly contact each other and are spaced from each other by a sensing gap Gs. When an external force is applied onto the second substrate 120 to make the deformation of the main spacer 128a greater than the sensing gap Gs, the first electrode 116 and the second electrode 126 which normally do not directly contact each other would be electrically connected to each other, so that a voltage variation on the first substrate 110 is present. The position of the pressing point can be obtained by detecting the above-mentioned voltage variation, converting the voltage variation into a signal by the system and further calculating the corresponding coordinates.
However, the prior art has following disadvantages. First, after pressing the touch display panel many times, the main spacer 128a and the sensing protrusion 128b may produce permanent deformations and the elastic restoring functions thereof get poor, which reduces and even eliminates the sensing gap Gs. Under the situation, the first electrodes 116 and the second electrodes 126 at some points directly contact each other, which is accompanied with poor touch function or touch function short, even damages the second electrode 126 over the sensing protrusion 128b. 
In order to increase the press-resistant performance and the lifetime of the touch display panel 100, main spacers 128a are deployed, but the scheme results in an increasing active force of touch leading to reducing the sensitivity of triggering touch inducting. On the other hand, the above-mentioned scheme likely produces low-temperature liquid crystal cells as well. So-called low-temperature liquid crystal cells are a sort of interspaces easily observed by a user through the display panel. Since the main spacer 128a and the liquid crystal layer 130 respectively have a different coefficient of thermal expansion (CTE), so that the two volumes of the main spacer 128a and the liquid crystal layer 130 are unable to be contracted in a same rate under a low temperature circumstance. As a result, the above-mentioned interspaces are produced in the liquid crystal layer 130 where the layer is supposed to be filled with liquid crystal molecules.
Second, since the main spacer 128a always contacts the first substrate 110 thereunder, the pixel electrode under the main spacer 128a must be spaced from the main spacer 128a by a distance to avoid a fault electrical connection between the second electrode 126 on the second substrate 120 and the pixel electrode to produce abnormal displaying due to an alignment error of assembling or a displacement of the main spacer 128a during pressing. In short, the conventional design would decrease the aperture ratio of the panel itself and thereby lower the transmittance or increase the cost of the backlight module.
Moreover, the second electrode 126 on the sensing protrusion 128b must contact the first electrode 116 on the padding structure for sensing 114b so as to change the potential of the first electrode 116 and thereby obtain the touch position, therefore, the thickness of the alignment film over the first electrodes would affect the active force of triggering touch inducting. In the prior art, the padding structure for sensing 114b is always formed by the combination of the stacking layers in the thin film transistor process (TFT process), therefore, the height of the padding structure for sensing 114b is unable to be effectively increased. When the alignment film is transfer-printed onto the first substrate 110 by using anastatic printing (APR) process, the thickness of the remained alignment film trace over the padding layer for sensing 114b can not be reduced with a height difference, which affects the uniformity and the active force of triggering touch inducting.
From the above described, it can be seen that how to overcome and prevent the above-mentioned shortages are considered as a significant project to be solved in the present production of touch display panels.